A bore air gauge is a precision measuring instrument that uses pneumatic pressure change to indicate bore diameter to very small tolerances. Air gauging is just the using that instrument to measure a bore more precisely than the old dial indicator type bore gauges can. Here's one for sale: http://www.marposs.com/product.php/e...tic_bore_gauge

I have used them to measure progress on a bore dia while running a Sunnen hone,but it has been many years ago.
You have a probe with a button on the end that is fed air.It is a clearance fit in the hole being measured.the clearance around the hole provides airflow and pressure drop.I have forgotten how the airflow was measured,whether by flowmeter,nanometer,or pressure gage,but the instrument would be calibrated with a ring gage of known size.The comparative air flow in the hole to be measured is a means to compare the diameters.
Particularly useful in a barrel is its ability to show tight and loose sections of a barrel throughout its length.The backpressure increases in tight spots and decreases on loose spots.

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